1. Field of the Invention
The present invention relates to a liquid crystal display device (hereinafter, referred to as an "LCD device") having a metal-insulator-metal structure.
2. Description of the Related Art
Recently, LCD devices have been used in products in a variety of fields including office automation and audio visual equipment due to excellent display quality, thinness, lightness, low power consumption and the long life thereof. Especially, the display quality of LCDs is one of the highest among various display devices designed for man-machine interface, and is even higher than that of CRTs. Among various LCD devices, demands for active matrix LCD devices are increasing remarkably as LCD devices satisfying higher standards of display quality are necessitated by larger screens and higher resolution.
One type of active matrix LCD device uses a three-terminal device, for example, a thin film transistor as a switching device. Production of this type of active matrix LCD device is complicated and requires at least six to eight steps of thin film formation and photolithography, thus raising production costs. LCD devices using a two-terminal nonlinear device as a switching device, which can be produced at lower costs than the devices including a three-terminal switching device and still have higher display quality than that of passive matrix LCD devices, have been developing rapidly.
One of the known two-terminal nonlinear devices is a device having a metal-insulator-moral structure (hereinafter, referred to as an "MIM device"). As is well known, an LCD device includes a bottom substrate having a pixel electrode and an MIM device, a counter substrate, and a liquid crystal layer interposed therebetween. In an LCD device using an MIM device as a switching device, response of the liquid crystal molecules to the voltage applied to the liquid crystal layer is steep. Due to such a characteristic, images having a high contrast ratio can be displayed even in high duty driving necessitated by a larger screen and a higher resolution.
An MIM device includes, for example, a lower electrode, an upper electrode, and an insulator film interposed between the two electrodes as is described in Japanese Patent Publication No. 61-32573. An MIM device having such a structure uses a tantalum thin film as the lower electrode as is described in the U.S. Pat. No. 4,413,883. As is described in Japanese Patent Publication No. 61-32674, an insulator layer of Ta.sub.2 O.sub.5 is formed by anodic oxidation Of the surface of the lower electrode. The upper electrode is formed of titanium, chrome, aluminum or the like on the insulator layer. In this manner, the MIM device is produced.
In such an MIM device, it is desirable that the current vs. voltage characteristic obtained when the current flows from the lower electrode to the upper electrode and the current vs. voltage characteristic obtained when the current flows from the upper electrode to the lower electrode should be symmetrical. Such a symmetrical current vs. voltage characteristic is obtained by forming the lower electrode and the upper electrode of an identical material with each other. However, if the two electrodes are formed of an identical material, the insulator layer and the lower electrode are possibly etched by a patterning process using photolithography for forming the upper electrode. Accordingly, if the two electrodes are formed of the same material, photolithography cannot be used to form the upper electrode. For these reasons, the upper electrode needs to be formed of a material which does not cause the insulator layer and the lower electrode to be etched by the patterning process for forming the upper electrode and still maintains the symmetrical current vs. voltage characteristic. For example, when tantalum is used for the lower electrode, titanium is used for the upper electrode.
FIG. 15 is a plan view of a part of an LCD device 100 including an MIM device 110. In detail, FIG. 15 shows a part of a bottom substrate 117 of the LCD device 100 corresponding to one pixel. FIG. 16 is a cross sectional view of the bottom substrate 117 shown in FIG. 15 taken along a line E-E' in FIG. 15. As is shown in FIGS. 15 and 16, the bottom substrate 117 includes a glass plate 108, a signal wire 101 formed of a tantalum film and located on the glass substrate 108, and a lower electrode 102 branched from the signal wire 101. An insulator layer 104 of Ta.sub.2 O.sub.5 located on the lower electrode 102 is obtained by anodic oxidation of a surface of the lower electrode 102. The lower electrode 102 may be a part of the signal wire 101, instead of being an area branched therefrom. An upper electrode 106 is located on the insulator layer 104. The MIM device 110 includes the lower electrode 102, the upper electrode 106, and the insulation layer 104 interposed between the lower electrode 102 and the upper electrode 106. A pixel electrode 107 formed of a transparent conductive film such as an ITO (indium-tin-oxide) film is electrically connected to the upper electrode 106.
An island 103 shown in FIG. 15 is formed of the same material as the signal wire 101 and by the same process. In other words, the island 103 is formed by patterning the tantalum film used for the signal wire 101. The island 103 prevents the upper electrode 106 from peeling off from the glass plate 108 and provides better, ohmic contact of the upper electrode 106 with the pixel electrode 107.
In FIGS. 15 and 16, the MIM device 110 is located directly on the glass plate 108. Instead, a base coat insulator layer may be provided on the glass plate 108 before the signal wire 101, the lower electrode 102 and the island 103 are formed. In this way, contamination of the MIM device 110 by the glass plate 108 is avoided.
FIG. 17 is a cross sectional view of the LCD device 100 including the bottom substrate 117 shown in FIGS. 15 and 16 and a top (or counter) substrate 118. In detail, FIG. 17 shows a part of the LCD device 100 corresponding to one pixel. In FIG. 17, a base coat insulator layer 109a is provided on the glass plate 108.
The counter substrate includes a glass plate 111, a base coat insulator layer 109b provided on the glass plate 111, and a counter electrode 112 located on the base coat insulator layer 109b and arranged in strips perpendicular to the signal wire 101. The counter electrode 112 is formed of a transparent conductive film such as an ITO film.
A liquid crystal layer 114 is interposed between the bottom substrate 117 and the counter substrate 118. Both surfaces of the liquid crystal layer 114 are respectively covered with alignment films 113. An assembly including the two substrates 117 and 118 and the liquid crystal layer 114 is interposed between polarizing plates 115 (outside the bottom substrate 117) and 116 (outside the counter substrate 118) with air gaps therebetween.
The LCD device 100 is produced in the following manner.
The base coat insulator layer 109a is formed on the glass plate 108, and a tantalum thin film which contains nitrogen doped thereinto and includes an .alpha. phase structure and a .beta. phase structure mixed together is deposited on the base coat insulator layer 109a by sputtering or the like at a thickness of 3,300 angstroms. The tantalum thin film is patterned as is prescribed by photolithography, thereby forming the signal wire 101, the lower electrode 102, and the island 103.
A surface of the lower electrode 102 is anodized to form the insulator layer 104 of Ta.sub.2 O.sub.5 having a thickness of 600 angstroms.
Titanium is deposited on the entire surface of the resultant lamination by sputtering or the like at a thickness of 4,000 angstroms and patterned as is prescribed by photolithography, thereby forming the upper electrode 106. In this manner, the MIM device 110 is produced.
ITO is deposited at a thickness of 1,000 angstroms and patterned, thereby forming the pixel electrode 107. In this way, production of the bottom substrate 117 is completed.
The counter substrate 118 is produced by forming the base coat insulator layer 109b on the glass plate 111, depositing an ITO film on the base coat insulator layer 109b at a thickness of 3,500 angstroms and patterning the ITO film as is prescribed to form the counter electrode 112 in strips.
The two substrates 117 and 118 are then each covered with the alignment films 113 for aligning liquid crystal molecules which will be interposed therebetween. The alignment films 113 are then rubbed. The alignment films 113 are formed of a polyimide organic polymer. A sealing material is applied to one of the substrates 117 and 118, and spacers are scattered on the other substrate. The two substrates 117 and 118 are assembled together and fixed to each other by heat and pressure.
A liquid crystal material including the liquid crystal molecules is injected between the two substrates 117 and 118 to form the liquid crystal layer 114 and sealed. Then, the polarizing plates 115 and 116 are provided outside the two substrates 117 and 118, respectively.
Various structures have been proposed for an MIM device used as a switching device in LCD devices. For example, Japanese Laid-Open Patent Publication No. 3-52277 discloses an MIM device in which side surfaces of the insulator layer obtained by anodizing the surface of the lower electrode are covered with another insulator layer. Japanese Laid-Open Patent Publication No. 2-304534 describes an MIM device in which the upper electrode and the insulator layer on the lower electrode are connected to each other through a hole formed in an inter-layer insulator layer formed on the insulator layer. Japanese Laid-Open Patent Publication No. 5-119353 discloses a structure in which a thick Ta.sub.2 O.sub.5 layer is formed at a shoulder of the signal wire by anodic oxidation or wet oxidation and a thin Ta.sub.2 O.sub.5 layer used for a current path is formed in the remaining area by anodic oxidation or dry oxidation. Such a plurality of Ta.sub.2 O.sub.5 layers used as upper electrodes each have a smaller width than that of the normal single signal upper electrode. In this structure, a plurality of MIM devices are provided for one pixel.
In the conventional LCD device 100 shown in FIGS. 15 through 17, the MIM device 110 is provided on the signal wire 101 or the lower electrode 102 branched therefrom. In such an MIM device 110, the current leak is excessively large from the side surfaces of the signal wire 101 and the lower electrode 102, and thus the withstand voltage is not sufficiently high. Accordingly, in production of such a conventional LCD device 100, rubbing of the alignment films 113 using a cloth generates static electricity, as a result of which, an excessive voltage is applied to the MIM device 110. In such a situation, insulation breakdown easily occurs. Even if insulation breakdown does not occur, static electricity possibly deteriorates the characteristics of the MIM device 110. Thus, defective MIM devices are produced by the influence of static electricity, lowering the production yield.
Further, an area of the upper electrode corresponding to the side surfaces of the lower electrode is easily exposed to stress and thus can easily be broken. As a result, the resulting MIM devices are defective, lowering the production yield.